1. Technical Field of the Invention
The present invention is directed to forming copper conductors on a PCB via a computer-controlled imagewise laser scanning without using phototools.
2. Prior Art
U.S. Pat. No. 3,469,982 to Celeste describes the prior art currently practiced to image PCBs via phototools, using a polymeric UV sensitive dry film usually .about.25 micron thick as the sole etch, or plating resist. It is the predominant method, because of its ease of application. The drawbacks reside in the high cost of generating phototools and the cost of manufacturing the dry film, limited resolution and laser imaging capabilities of the dry film, due at least in part to its heavy thickness of over 25 micron, and the environmentally unfriendly disposal features inherent in the manufacture and use of dry films.
The industry is making strenuous efforts to develop DW systems and thus obviate the need for phototools. Such efforts are abundantly reflected in the wealth of prior art literature, both in the form of patents and publications. The following published articles are referenced herewith as typical and pertinent: (1) Emerging technologies, PC fabrication, volume 18, No. 9, September 1995; (2) Laser Direct Writing of Copper on Various Thin-Film Substrate Materials, Applied Surface Science 46 (1990) 143-147, Elsevier Science Publishers B.V. (North Holland); Kuchta, Technological Requirements for Direct Imaging, Technical Paper No. 1, A 8/1, PC World Conventions, June 1990.
The contents of U.S. Pat. No. 5,328,811 is included by its reference. The method it describes has yet to surface in the industry. The patent teaches DW imaging via thermal laser energy that causes a redox reaction between two "reagents" applied on a metallic surface, such as the copper cladding of an unimaged PCB panel. Typically, the patent discloses the redox reaction of CuO with a reducing layer such as polyaldehyde, when impacted with the heat of a thermal laser. Areas that have been scanned with the thermal laser will cause the CuO to be reduced to copper metal, thereby allowing the copper to be selectively etched where exposed. Essentially, the CuO acts as etch resist or plating resist, wherein perhaps lies the reason for the difficulties that can be expected when attempting to practice the teachings of above-cited U.S. Pat. No. 5,328,811. Indeed, CuO cannot be relied upon to safely serve as a reliable etch resist, whether copper etching is done in ammoniacal or acid-based etchants. In the above cited patent, copper oxide or sulphide acts also as absorber of heat generated by the thermal laser. In some examples of above cited patent, etch resistance of copper metal can be aided by the reducer film covering the underlying copper oxide or sulphide. But the thrust of U.S. Pat. No. 5,328,811 is a redox reaction initiated by thermal laser between a first and second "reagent" overlying the copper cladding, and its application is therefore limited to thermal lasers. Herein perhaps lies the difficulty in its achieving industrial success, since such redox reactions are relatively slow and do not generally enable sharp image delineation.